1. Field of the Invention
The present invention relates to a circuit substrate and a process for producing it. More particularly the present invention relates to a circuit substrate used in optical instruments such as cameras, home electric appliances, computers, word processors, measuring devices or the like, and a process for producing such a circuit substrate.
2. Related Background Art
Conventional processes commonly used to produce circuit substrates comprise the steps of (1) forming a copper foil circuit pattern on an insulated substrate by photolithography, or by screen printing using a conductive paste, (2) thereafter coating a solder resist or the like on the surface of the circuit to provide thereon an insulating layer, (3) subsequently coating a conductive paste on the surface of the insulating layer to provide thereon a shielding layer, and (4) further coating a solder resist or the like on the shielding layer to provide thereon an insulating layer. Methods for electromagnetic wave shielding may include, in addition to a method of providing the above shielding layer, a method in which shielding is effected by surrounding a circuit substrate with a sheet metal. The insulating layer on the circuit pattern can also be provided by laminating a dry film.
The conventional processes for producing circuit substrates, however, provide an insulating layer having a small thickness at the corners of copper wires that constitute a this arrangement results in a break of the insulating layer at the parts corresponding thereto. The conventional processes for producing circuit substrates also tend to cause a dielectric breakdown or defective insulation because of inclusion of bubbles or the like between copper wires, exacerbating the problem. In addition, the conventional processes for producing circuit substrates require complicated production steps which take a long time for their manufacture, also bringing about a problem in cost.
As for the conventional method in which shielding is effected by surrounding a circuit substrate with a sheet metal, it requires a broad space in order for the metal sheet to be provided, and hence has been not suitable for making products small-sized.